1. Field of the Invention
The present invention relates generally to high temperature, thermoset polymers. More particularly, the present invention relates to new ethynylated phenylbenzimidazole compounds, their use as prepolymers and their fabrication into polymer structures which are stable at high temperatures.
2. Description of the Background Art
In the past two decades there has been a great deal of interest in developing heterocyclic polymers which have sufficient thermo-oxidative stability to be useful at temperatures up to 371.degree. C. (700.degree. F.). Although many types of these high temperature polymers have been developed, most of them have not been sucessfully utilized in the production of hardware or produced on a commercial scale. The lack of commercialization of these polymers is due mainly to difficulties in processing. Generally, such polymers are highly intractable and cannot be effectively used in fabrication. Even when made in prepolymer form, they still pose the problem of generally liberating gaseous by-products when they undergo further polymerization by condensation.
Five typical polymers of the general type mentioned above are: poly[benzobis(triazolo)phenanthroline]; poly[(7-oxo-7,1OH-benz[d,e]imidazo[4',5':5,6]benzimidazo[2,1a]isoquinoline -3,4,10,11-tetryl)-10-carbonyl] (commonly called BBL); poly[bis(benzimidazo)benzophenanthroline] (commonly called BBB); polybenzimidazoquinazoline (generally referred to as PIQ); and polynaphthalimide. These materials represent some of the most stable polymers known. Other heterocyclic polymers such as polybenzoxazoles, polybenzimidazoles, and polybenzothiazoles are also well documented in the polymer literature and have been shown to have excellent thermal stability.
The preparation of thermosetting prepolymers of the above-discussed general classes of materials and the development of addition curing mechanisms for their polymerization could eventually provide useful commercial products. Their thermal stabilities are sufficiently close to each other that it would be very difficult to predict accurately which class would ultimately give the most useful thermoset resins, since many other variables must be considered. Accordingly, there is a continuing need to develop additional monomers which can be conveniently processed by commercial fabrication techniques into polymers which exhibit the high temperature stability of the above-mentioned compounds.
Various acetylene-substituted compounds have been shown to homopolymerize by the application of heat or a catalyst to form resins which are stable at high temperatures. As a result, practical thermosetting acetylene-substituted prepolymers have begun to appear in the literature. These acetylene-terminated prepolymers have shown promise since they provide high-temperature thermosets which cure without the evolution of gaseous by-products and could eventually yield fabricated structures having excellent thermal and mechanical properties. However, the early introduction of an acetylene-substituted resin, Haveg-H-resin (Hercules, Inc.), met with very limited success, mainly because of the relatively small processing window and the poor long-term high-temperature oxidative stability of the cured product.
Thermosetting polyimide oligomers have also been considered as possible useful high temperature resins due to the excellent thermal stability inherent in the polyimide backbone. A series of acetylene-terminated polyimide prepolymers has been developed which is soluble in an acceptable common solvent and tractable in imidized form. These prepolymers were subsequently marketed as Thermid 600 by Gulf Oil and Chemical Company.
Other acetylene or ethynyl terminated oligomers such as acetylene-terminated phenylquinoxalines have been under development. These prepolymers are also thermosetting and can be cured to provide moisture-resistant products of very high thermo-oxidative stability. The oligomers have many properties desirable for processing, i.e., high solubility in low-boiling-point aprotic solvents, low softening temperature prior to cure, a wide temperature difference between the melting and cure temperatures, cure without the evolution of gaseous products, and after cure, glass transition temperature sufficiently high (310.degree.-350.degree. C.) to allow long-term use at 250.degree. C. Relative advantages of the various types of acetylene-terminated polyquinoaxlines have not been fully determined, although mechanical property data have been collected.
Another acetylene-terminated prepolymer which is receiving considerable attention is bis[4-(3-ethynylphenoxy)phenyl]sulfone. This material has been evaluated as a polymerizable plasticizer for various thermoplastics.
As is apparent from the discussion above, numerous different acetylene-terminated monomers have been developed which are suitable in varying degrees for use in fabricating high temperature resins. Even so, there is still a continuing need to develop new classes of acetylene-terminated compounds which are easily and conveniently fabricated to provide addition-curing processible matrix resins having a service temperature capability of 350.degree. C.